Generally, torches of the type to which this invention relates supply an ionizable gas to the torch nozzle in front of a negatively charged electrode. A high d.c. voltage is applied to the electrode. A high voltage, high frequency AC signal is superimposed on the DC signal causing a spark to jump between the electrode and the torch nozzle which establishes an initial starting pilot arc. The pilot arc heats and ionizes the gas passing through the nozzle and establishes a plasma flow. When the nozzle of the torch is brought towards the workpiece, which has a lower impedance path than the torch nozzle, the pilot arc jumps from the nozzle to the workpiece establishing a plasma arc between the electrode and the workpiece. The plasma arc permits the torch to perform its appropriate function i.e., either cutting, welding or metal deposition.
In all plasma arc torch systems a nozzle is provided to enclose and shield the electrode (which is at a high voltage) against accidental grounding, to direct the flow of ionizable gases as a plasma against the workpiece and to constrict the plasma column giving a very high plasma temperature. Torch parts, in particular the torch nozzle, are "somewhat" consumable and must eventually be replaced. In particular, the pilot arc between the nozzle and the electrode will eventually wear away the nozzle. Also contributing to the wear of the nozzle is the heat from the ionized gases and to some extent the adverse effects of the heat eventually requires the electrode to be replaced. Accordingly, the nozzle and electrode are usually provided as replaceable parts which are threaded into the torch body so that the nozzle shields and encloses the electrode except for a small orifice through which the plasma arc passes. When the parts are to be replaced, they must, in fact, be replaced in a proper assembled relationship to one another to assure that the electrode is properly shielded to avoid accidental grounding which could occur if the torch was inadequately assembled without the nozzle present.
The prior art has recognized this problem and has proposed various solutions. For example, U.S. Pat. No. 4,663,515 to Kneeland et al. provides a nozzle which, if not in place, allows the gas pressure to rise. The rise in gas pressure is sensed and the power source disabled. U.S. Pat. No. 4,590,354 to Marhic and 4,682,005 to Marhic disclose protective nozzles surrounding the electrode and slideable into working contact by the pressure of the plasma gas. While such devices will disable the torch, in practice by the time the gas pressure is sensed or activated an arc can already be discharged. In addition, gas pressure sensing devices are expensive.
In U.S. Pat. No. 4,585,921 to Wilkins et al., incorporated herein by reference, a switch contact is established when a cone surrounding the nozzle is positioned in place. Unless the switch is activated, the power source is disabled. However, in the Wilkins device, the cone could be in place and the nozzle inadvertently deleted from the assembly. Thus the switch could be activated because the cone is in place but the electrode could be exposed. There thus remains a need to prevent any inadvertent arc discharge from a plasma arc torch unless the parts of the torch are assembled properly.
In all plasma arc torch systems, the main source of electrical power is transmitted to the electrode in the torch body by a main conductor encased within a cable which extends from the torch body to the power supply. The cable is susceptible to being punctured or severed by any one of a number of different metal objects which are always present within the confines of a shop environment. Such metal objects can contact the main conductor establishing the same type of electrically hazardous condition which could otherwise exist by an exposed electrode. Heretofore, attempts to address such problems have been directed towards producing a tough, durable and puncture resistant cable. Such cables increase the expense of the system and require periodic inspection and replacement.
Inherent in all plasma arc torch systems using a pilot arc, is the fact that the current in the arc increases momentarily when the pilot arc is transferred from the nozzle to the work as a plasma arc. When the arc transfer occurs, the arc momentarily travels from the electrode to the nozzle orifice adjacent the electrode and from the nozzle orifice to the work. This double arc deteriorates the nozzle eventually resulting in a larger nozzle orifice and exposing the electrode. The prior art has recognized this problem and has proposed various power circuits which decrease the current in the arc at the moment of transfer. One example of such a circuit is shown in U.S. Pat. No. 3,745,321 to Shapiro et al., incorporated herein by reference. While such circuits are somewhat responsive to the problem, there are either stability problems present or energy efficiency drawbacks associated therewith.
Apart from plasma arc torch systems, per se, the assignee of the present invention has utilized in its arc welding power source, a fault detect circuit which senses a short between the electrode and the nozzle and in response to a sensed voltage in excess of a predetermined value, disables the power source.